Audio amplifiers for increasing the electrical power level of audio frequency input signals without distorting them are required in many applications. In particular, stereo decoder amplifiers are sometimes required in quality stereo equipment for amplifying the signal at the output terminal of the stereo decoder and applying it to a deemphasis filter network including a resistor and capacitor having values set by the industry. Such stereo decoder amplifiers are required to have a high output impedance so that they do not undesirably affect the filter characteristics. A buffer amplifier is usually connected to the output terminal of the deemphasis filter so that the load also does not affect the time constant of the filter. In solid state circuits, this buffer amplifier has usually been in the form of an emitter follower circuit which tends to undesirably distort the filtered output signal.
When the composite stereo signal is indicating a center location for the sound source, the left channel (L) and right channel (R) decoder output signals have approximately the same wave forms. Also, under these conditions the decoder output signals have abrupt transitions. Unless the decoder amplifier has high positive and negative slew rates, so that the output signal thereof is able to follow these input signals, the ouput signal of the amplifier will be of reduced amplitude and contain distortion. The reduction in amplitude is caused by the slow switching in the decoder amplifier which reduces the magnitude of the energy transferred. The distortion can be caused by asymmetric rise and fall times of the response characteristic of the decoder amplifier which produces undesirably different amounts of energy transfer to the load during the positive and negative excursions of the driving signal. For instance, if the amplifier responds more quickly to the positive transitions than to the negative transitions then the average of the energy transfer is greater for positive going signals than for negative going signals, which results in second harmonic distortion in the decoder amplifier output signal. Therefore, it is important for such amplifiers to have a high enough slew rate so that it can adequately respond to the abrupt transitions of the signal at the decoder output. Moreover, the decoder amplifier should have a linear transfer characteristic so that is does not create other distortion in the signal passing therethrough.
Monolithic stereo decoder integrated circuit amplifiers have sometimes included a single stage PNP "turn-around" transistor. Such circuits operate adequately if no gain is required of the amplifier. However, if even a gain of two is desired, the PNP device is required to conduct an undesirably large amount of current which causes its beta characteristics to become unpredictable and nonlinear, or to be undesirably large. Alternatively, if the quiescent current through such devices has a low magnitude, designing the circuit to provide any gain causes an undesirably large change in the base-to-emitter voltage due to large dynamic output signals which tend to cause a nonlinear relationship between the output current and the input voltage.
Consequently, other more complicated prior art stereo decoder amplifiers have been developed which utilize differential amplifiers having current sources and turn-around circuits. Such circuits are undesirable because of their complexity which results in increased chip cost as a consequence of the large amount of area required by the amplifier and because of their limited output swings due to required bias voltage drops across them. In particular, the current sources thereof require a plurality of devices which generally must be connected between the positive and negative power supply conductors. The complexity of the layout of these circuits reduces their attractiveness. Also, prior art stereo decoders include frequency compensating capacitors which sometimes tend to prevent them from rapidly turning off which tends to reduce the negative slew rate and distorts the stereo signals.